Monolithic bodies having longitudinal through passages are produced by forcing an extrudable plastic mixture of ceramic material through interconnecting die passages in which the extrudable material is shaped and coalesced into a structure of intersecting, relatively thin walls that define the passages of a honeycomb structure having a cell density which is established by the number of passages formed through a cross section of a unit of the honeycomb structure. In one application, the material is a mixture of materials, such as clay, talc and alumina, that can be fired to form cordierite or other heat-resistant ceramic.
The main purpose of such honeycomb-type structure is to provide an extended surface area for exhaust gases to pass over. The surface area is increased by increasing the cell density, e.g., the number of cells per square inch through which the exhaust can flow. At the same time, it is usually necessary to form thinner cell walls of uniform thickness.
In certain cases the monolith consists of many longitudinal passages, as many as 400 per square inch of cross section of the monolith. In the extrusion process a wet or green ceramic precursor mixture of a suitable length is formed. It is directed through an extrusion die to form a resultant honeycombed green extrusion. The extrusion is sliced cross ways to form several pieces which are dried and fired to form the monolith substrate. The internal walls of the substrate are then coated with a noble metal catalyst finely dispersed on a suitable wash coat. Such honeycombed substrates are oval or elliptical in cross-section, and the individual cell cross-section is usually generally of square cross-section. The greatest dimension of the overall cross-section can be on the order of six inches, and the length to the monolith can be about six inches The cell dimensions, however, are several orders of magnitude smaller, e.g., in the range of 0.044 inch square at the cell hole and with cell wall thicknesses on the order of 0.006 inch. In order to extrude monoliths of this magnitude cell size at desired densities on the order of 400 cells per square inch and uniformly thin wall thicknesses, it is essential that the extrusion die be a precision die.
An example of such a die is set forth in U.S. Pat. No. 4,486,934 issued Dec. 11, 1984 to James R. Reed. The Reed design represents a common prior art extrusion die which, while suitable for its intended purpose, is of a configuration and is made by a method which is very complicated to form when bodies having 400 cells per square inch are to be extruded therethrough. Use of known precision machining methods to form such high cell density thin wall extrusion paths in prior art extrusion dies involves considerable expense and difficulty.